Electrostatic printing



Sept. 4, 1962 H. G. GREIG ELECTROSTATIC PRINTING Filed Oct. 1, 1953 2 Sheets-Sheet l Pic. 1 a

Fig. 2

IN VEN TOR.

.m M e p w A G Sept. 4, 1962 H. G. GREIG ELECTROSTATIC PRINTING 2 Sheets-Sheet 2 Filed Oct. 1, 1953 Tom United States Patent fifice 3,052,539 Patented Sept. 4, 1962 3,052,539 ELECTROSTATIC PRINTING Harold G. Greig, Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 1, 1953, Ser. No. 383,677 22 Claims. (Cl. 961) This application is a continuation-in-part of my previously filed application, Serial No. 248,937, filed September 29, 1951, and now abandoned.

This invention relates to improvements in electrostatic printing and copying. More particularly, the invention relates to an improved electrostatic printing process and to improved apparatus and recording elements which may be utilized in the process.

By an electrostatic printing process is meant generally that type of process for producing a visible image, reproduction, or copy, which includes, as an intermediate step, converting a light image or signal into an electrostatic charge pattern on a carrier base. The process may also include the converting of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be of different size. On the other hand, the visible image may be a series of characters corresponding to a signal received from some external source.

Previously proposed electrostatic printing processes have usually included the steps of providing a conductive backing plate with a surface layer of a photoconductive insulating material such as selenium, anthracene, or sulfur with the selenium being preferred, giving the surface layer an overall electrostatic charge, focussing a light image on the charged surface in order to discharge the portions struck by light rays While leaving the remainder of the surface in a charged condition thereby forming a charge image, developing the charge image by application of an electroscopic powder, transferring a copy of the developed image to a sheet of paper or other material upon which the reproduced image was desired, and fixing the last named image.

Using the type of process above described, sharp reproductions can be obtained of line drawings, typewritten or printed matter, and the like. The process also has the further advantages of simplicity of apparatus required and cheapness and ease of handling of the developing materials. However, the process has some inherent disadvantages which it would be desirable to eliminate. For example, in order to make a single final copy on paper, it is necessary to first make a transferable copy on the photosensitive plate. This is somewhat tedious and time consuming where a large number of separate items are to be copied. Moreover, the photosensitive plate must be subjected to a special cleaning process to get rid of the intermediate print each time, before the plate can be used again. And, after a relatively few uses, the photosensitive material becomes fatigued and either must be permitted to rest for a considerable period or must be regenerated. Thus, where a large volume of work is to be handled, unless a method of regeneration is used, a number of photosensitive plates must be rotated in use. The photosensitive surface is also subject to wear in handling and use and must be repaired or renewed from time to time.

It will therefore be apparent that, at least in making single copies of items, it would be highly desirable to employ a process in which the intermediate step of using a photosensitive plate having a conductive backing could be dispensed with entirely, that is, a process having all the advantages of simplicity and cheapness of material inherent in electrostatic printing processes and, at the same time, involving direct reproduction on the final surface.

The process of the present invention is an electrostatic reproduction process in which the necessity of using an intermediate photosensitive plate for forming a preliminary image and transferring this image to paper or other base material, is eliminated. The process utilizes an improved recording element which may comprise a base sheet of either a relatively good electrically conducting material, such as metal, or a relatively poor electrical conductor, such as paper. The base sheet carries a composition comprising a photoconductive zinc oxide suspended in an electrically insulating, film forming vehicle. The photoconductive zinc oxide should have a surface photoconductivity higher than 10 ohms- /square/watt/ om. when exposed to a wavelength of about 3900 A. An overall electrostatic charge is applied to at least a portion of an active surface of the recording element and a light image of the subject to be reproduced is projected upon the charged portion causing a charge pattern or image, corresponding to the light image, to be formed. This charge image may be stored for a time in the dark or it may be immediately developed to render it visible. Development may be carried out by applying an electroscopic developing substance, such as a pigmented powder, capable of adhering to the charged portions and not to the discharged portions. Alternatively, the developer substance may be one which adheres to the edges of the discharged portions of the charge image.

One object of the present invention is to provide an im-- proved process of copying printed or typewritten matter.

Another object is to provide an improved single color reproduction process.

Another object is to provide a simplified method of electrostatic printing.

Another object of the invention is to provide a simplified method of reproducing an image composed of contrasting lights and shadows in which method the use of a photosensitive intermediate image carrier is eliminated.

Another object of the invention is to provide an improved method of preparing a carrier base to receive an electrostatic charge pattern.

Another object of the invention is to provide an improved direct method of electrostatically prinitng on a carrier base such as paper or metal.

Another object of the invention is to provide an improved surface for receiving an electrostatic charge pattern.

A further object of the invention is to provide improved surfaces for receiving an electrostatic charge pattern or image which can be developed to produce a visible image.

Still another object of the invention is to provide an improved process for converting an electrical signal into visible recorded information.

These and other objects will be more apparent and the invention will be more readily understood from the following detail description and the accompanying illustrative drawing of which:

FIGURE 1 is a perspective view of apparatus which may be utilized in carrying out the charging step of the process of the present invention,

FIGURE 2 is a View partially in perspective and partial-ly in section, of apparatus which may be utilized in forming a charge image on a carrier sheet in accordance with the process of the present invention,

FIGURE 3 is a perspective view of simple means for developing a charge image formed as shown in FIGURE 2, and

FIGURE 4 is a perspective View of a sheet carrying a developed image and means for fixing the developed image.

FIGURE 5 is a diagrammatic elevation view of apparatus of the present invention suitable for carrying out a continuous reproduction process in accordance with the present invention.

Important features of the present invention are the provision of an improved recording element for electrostatic printing and utilizing this element to provide an improved electrostatic printing process.

The improved recording element comprises a base sheet which may be either a relatively good electrical conductor, such as metal, or a relatively poor electrical conductor such as a cellulosic sheet having a calendered surface, for example paper or cellophane. The base sheet may be coated or, if adapted to impregnation, may be impregnated with a particular type of composition. The composition comprises a powdered photoconductive zinc oxide suspended in an electrically insulating, film forming vehicle.

In order to determine Whether or not a particular zinc oxide is suitable for use in the present invention, a test of its photoconductive properties may be made as follows. A small quantity of the zinc oxide is reduced to a powder, the powder is compressed under high pressure, i.e., about 15,000 lbs. per square inch to form a pellet, electrodes, as of silver paste, are applied on a surface of the pellet leaving a square area of surface uncoated, the pellet is placed in a monochromator with the aforementioned uncoated surface area facing the light source, and successive wavelengths of light throughout the spectrum are projected on this surface. -A D.-C. potential is placed across the electrodes and the current flowing between the electrodes is measured as a function of wavelength with the intensity of radiation being held constant.

The zinc oxides which are suitable are those which are substantially electrically non-conductive in the dark. When exposed to light, they should have a surface photoconductivity of a certain level in order to be of practical use in the present process. In testing zinc oxides in order to determine their suitability and utilizing a pellet form, it is convenient to express the results of the measurements of the test as a surface photoconductivity because substantially all of the light is absorbed within a thin layer at the surface of the pellet. It has been found that, to be useful in the present invention, the zinc oxide selected should have a surface photoconductivity of at least about ohm- /square/watt/cmfi when exposed to a wavelength of about 3900 A.

Having established the threshold value of surface photoconductivity needed in the process, it is possible to test any zinc oxide otherwise suitable from the standpoint of stability, compatibility, dark resistivity, color, etc., in order to'determin'e whether it can be used. Zinc oxide can be prepared in a white form or an orange-pink form, hereinafter referred to as the pink form.

Of the various white zinc oxides commercially available not all have a surface photoconductivity greater than about 10- ohm- /square/watt/cm. at a wavelength of about 3900 A. which is about the wavelength of peak photosensitivity for this material, but many pure grades of ZnO made by a dry process have been found suitable. If the percentage of impurities is too high, inferior results are usually obtained.

The electrically insulating, film forming material may preferably be any one of a number of substances. Most desirable is a synthetic resinous material having a relatively high dielectric constant and high dielectric strength. Examples of suitable resins are polyvinyl acetate, copolymers of vinyl chloride-vinyl acetate, polystyrene, and silicone resins. Other resin-like materials such as cellulose ethers and cellulose esters may also be used. Suitable examples of these types of materials are methyl or ethyl cellulose and cellulose nitrate. Natural resins are also suitable, shellac being but one example. In addition to resinous materials, it has also been found that various waxes may be used. Examples of suitable waxes are paraflin and carnauba wax but others may be substituted, if desired.

Besides being electrically insulating and filmdorrning, the binder must be water-insoluble and should be sufficiently flexible, when in the form of a thin film, to be practical as a coating for paper.

The usual plasticizers for the resins mentioned above may be used to increase the flexibility of the coating. The chief requirement is that the plasticizer should not impair the dielectric properties of the coating to any appreciable extent.

The zinc oxide powder may be suspended in the vehicle in either one of two Ways. The simplest way is to dissolve the film-forming material in an organic solvent capable of efiecting solution and then mix in the Zinc oxide powder. Alternatively, the zinc oxide may be dryblended by kneading with the film-forming ingredient heated to a sufficiently high temperature to render it plastic. In the case of the waxes, they may be melted and the zinc oxide mixed with the melt.

A preferred example of coating composition will now be given.

Example 1 60 gms. of a 10% solution of polyvinyl acetate resin in methanol 40 gms. white zince oxide, C.P. (made by a dry process) 20 ml. of acetone as thinner These ingredients were mixed for 1 hour in a porcelain ball mill to obtain a smooth, uniform mix. Ordinary white bond paper was coated and impregnated by immersion in the mix, removing the impregnated paper, draining off excess composition and air drying. The mix may also be applied as a surface coating by thinning with acetone and spraying with a spray gun, or by any other of the usual paper coating techniques such as flow coating, roller coating, and the like.

Example 2 9.6 gms. silicone resin (GB. No. 81,182), alkyl-aryl 22.5 gms. toluene 24 gms. white zinc oxide, C. P. (made by a dry process) A mix was prepared of these ingredients as in Example 1 and a sheet of paper was coated by flowing the mixture over one of its surfaces.

Example 3 A mixture of alkyl-aryl type silicone resin (Dow- C-orning No. 803), toluene, and white zinc oxide was prepared with the ratio of resin to zinc oxide being 5 to 1. The mixture was thinned with acetone and sprayed on a surface of a sheet of paper.

Example 4 Equal parts by weight of white zinc oxide and melted parafiin were mixed and flowed over the surface of a sheet of paper to form a smooth, uniform coating.

Example 5 White zinc oxide and melted carnauba wax were mixed m a ratio of 3 to 1 by weight. The molten mixture was flowed over the surface of a sheet of paper.

Example 6 Carnauba wax was dissolved in toluene and white zinc xide was mixed with the solution such that the ratio of zinc oxide to wax was 3 to 1. This mixture was applied to a sheet of paper by brushing.

Example 7 Methyl cellulose was dissolved in a solvent and white zinc oxide was mixed with the solution such that the ratio of Zinc oxide to methyl cellulose was 2 to 1 by weight. This mixture was spread evenly over the surface of a sheet of paper.

Example 8 To a by weight solution of polystyrene in toluene suflicient white zinc oxide was added to make the ratio of zinc oxide to polystyrene 8 to l by weight. This mixture was thinned with toluene and used to apply a smooth coating to paper or metal sheets.

Example 9 A 10% solution of vinyl chloride-vinyl acetate copolymer resin in acetone was prepared and sufiicient white zinc oxide added to make the ratio of zinc oxide to resin about 7 to 1 by weight. This was thinned with more acetone and applied to sheets of paper.

Example 10 10 gm. dry shellac was dissolved in 180 cc. methanol and cc. acetone. 100 gm. of this solution was then mixed with 50 gm. white zinc oxide. This mixture was used to coat paper and metal.

Example 11 40.3 gm. /2 sec. nitrocellulose was dissolved in a solvent made up of 278 gm. toluene and 185.7 gm. ethyl acetate. To 180 gm. of this solution was added 80 gm. white zinc oxide. This mixture was used to coat paper.

In these composition, it has been found possible to vary the ratio of zinc oxide to the fihn-forming vehicle (in the dry state) between about 1 to 1 and about 12 to 1. Any solvent for the film-forming ingredient may be used. The optimum quantity of zinc oxide for any given composition will depend upon the particular filmforming ingredient used since these vary considerably in electrical properties such as dielectric constant. It has been found, unexpectedly, that relatively pure zinc oxide, such as that designated C.P. or U.S.P. gives much better results in the present process than ordinary technical grade zinc oxide commonly used heretofore in pigments or in coated papers. Although the reason is not understood, zinc oxide having a very low concentration of impurities, when combined with the type of vehicle described herein, is capable of taking and holding an electrostatic charge in the dark and of being discharged in the presence of light. The vehicle is also an essential part of the composition. Without the vehicle, the zinc oxide does not exhibit the properties necessary for successfully carrying out the present process.

Herewith are given chemical analyses of impurities found in three typical samples of zinc oxide which were found suitable for use in the process of the present invention.

Made by dry Made by wet process process No. 1, No. 2, No.3, percent percent percent The physical form of the zinc also has an effect on the optimum amount of this ingredient. There is an appreciable difference in density between Zinc oxide powders prepared by wet and dry processes.

White zinc oxide is a preferred photoconductive substance for use in the processes, coated base materials and apparatus of the present invention for a number of reasons. The fact that it is white causes it to be more acceptable for certain uses such as reproducing photographs. It is also relatively abundant and cheap, stable, compatible With synthetic resins and easy to apply as a coating on paper and other bases.

However, it has also been found that a pink form of zinc oxide exhibits desirable photoconductive properties similar to the white zinc oxide but is of the order of several times faster with regard to length of exposure required for incandescent light. Moreover, the pink zinc oxide is responsive to light throughout most of the visible spectrum whereas the white zinc oxide has its peak response in the ultra-violet range.

One method of manufacturing a pink zinc oxide suitable for use in the present invention is described and claimed in US. patent application, Serial No. 329,473, filed January 2, 1953, Conn et al., assigned to Merck and Company, Inc. The process described in the said application comprises heating ammoniated zinc carbonate at a temperature of -400 C. Preferably, the carbonate is heated at 250 C. for about 1-2 hours or until all of the ammonia and carbon dioxide are released. At this point a weight loss of about 43% will be realized and the resulting product will have a particle size of about 100200 A. The ammoniated zinc carbonate may be first prepared by adding white zinc oxide to a solution of ammonium carbonate in 28% ammonia water, blowing in carbon dioxide until no further separation of solid occurs, filtering out the solid, which is the ammoniated Zinc carbonate, washing and drying.

The pink zinc oxide, prepared as above described, may be substituted, weight for weight, for the white zinc oxide given in the examples and the compositions suitable for use are also otherwise the same as those shown in the examples.

The coated or impregnated and dried paper, prepared as above described, is then utilized as follows to receive an image thereon.

Referring now to FIG. 1, the coated paper 1 is provided with an electrostatically charged layer 2a over one of its surfaces 2 by passing it, in the dark, past an electrostatic charging device 3. The charging device may comprise an array of fine wires 4 mounted near a grounded metal plate 5. A source of high voltage D.C. (not shown) is connected to the wires so as to give them a negative charge with respect to the ground plate. The charge is of sufficiently high voltage to cause a corona discharge adjacent the wires. The paper is placed in contact with the ground plate and passed through the corona discharge and thus becomes charged negatively. It is, of course, understood that the apparatus shown in FIG. 1 must either be enclosed in a light-tight box or must be used in a darkened room.

The next step in the process is to discharge selected parts of the charged surface of the paper in order to obtain a charge pattern. Referring now to FIG. 2, this may be done by removing the paper from contact with the grounded plate 5 and exposing it to a light image from a suitable source. This source may, for example, be a sheet 7 containing black and white printed matter 8 or it may be a typewritten sheet or a line drawing. This print may be focussed onto the charged surface 2a with the aid of an optical system 9 and using light from one or more incandescent bulbs 10 or other light source depending upon the wavelengths to which the particular photoconductor used is most sensitive. The sheet containing the material to be copied may be placed in one compartment A of a light-tight box and the sheet which is to receive the print may be placed in another separate compartment B of the box. The subject to be copied may, on the other hand, be projected onto the charged surface by light placed behind a photographic film, where the subject is an image on the film. The light image may also be projected by scanning a group of letters, characters, and figures in the form of a stencil, with a light spot from a cathode ray tube, or the spot from the cathode ray tube may be merely scanned across the charged surface in response to an electrical signal received from an external source.

7 Wherever the light strikes the charged coating layer 2a, the electrostatic charge is lowered or removed. If decreased, the decrease is in proportion to the intensity of the light. This leaves a pattern of remaining charges 11 corresponding to the dark portions 12 of the image.

The charge image may be stored for a time, if desired. Ordinarily, however, the next step in the process is to develop the charge image to render it visible. This may be accomplished by maintaining the sheet in a darkened condition and, as shown in FIG. 3, dusting the charge image 11 with an electroscopic developer powder 13.

'A suitable developer powder is one of the type disclosed and claimed in the copending application of H. G. Greig, Serial No. 187,827, filed September 30, 1950. A suitable example of this type of developer powder may be obtained by taking a fine powder having good dielectric properties, such as sublimed sulfur, forming it into a thin slurry or paste by mixing 40 parts by weight of the sulfur with 50 parts by weight methanol. 1.5 parts by weight of spirit-soluble nigrosine dye SSB, Color-Index No. 864, is added to the paste and the mixture stirred until the dye is completely dissolved in the alcohol. To this is then added 1 part by weight of Iosol Black or any other dye which will react with the nigrosine to precipitate a black dye. The product is isolated by filtering, washed with a small amount of methanol and dried. This powder can either be used as is, or after grinding in a ball mill, to enhance its electroscopic properties.

Instead of merely dusting the powder, it may be applied to the image by mixing it with small glass beads and causing the beads to flow over the surface of the paper. The beads serve as a carrier for the powder.

The typeof powder above described is a positively charged powder and will readily adhere to negatively charged areas of a charge image. This type of powder is preferably used with a coated surface prepared as in Examples 2, 3 and 4 since, using these types of coated surfaces, the powder will adhere to the dark areas of the image. On the other hand, negatively charged powders can also be used in the present process since they will adhere to the edges of the areas exposed to light. Suitable negatively charged powders are dyed or pigmented powdered synthetic Or natural resins. Examples of synthetic resins are coumarone-indene resins and Vinsol. Examples of natural resins are gum copal and rosin. This type of powder may be used with coated surfaces prepared as in Examples 1 and 5-10, for example.

The type of developed image 14 obtained using the precipitated dye-treated sulfur powder described above is a positive. That is, the dark portions of the developed image correspond to the dark portions of the original. Using negatively charged powders, a negative print is obtained.

When the paper is coated or impregnated so that a coated surface is obtained on both sides of the sheet, a print can be made on each side by repeating the printing procedure on the reverse side after the first print is made.

The developed image 14 on the coated paper may be fixed so that it does not smear in handling. With a developer powder having a base of low melting point, the image can be fixed by heating to fuse the powder to the surface. Heating can be done with an infra red heating lamp 15, for example. The dyed sulfur or dyed synthetic resin powders can be fixed in this way. Other types of developer powders may be fixed by spraying with a lacquer or with shellac.

Although the process has been described in connection with printing on a paper base, it works equally well if the coating material is applied to abase which is electrically conductive, for example, aluminum sheet, metal foil, or paper loaded with carbon black.

The process of the present invention, as above described, has a number of advantages. A visible image can be placed directly on an insulating material such as paper with no conductive backing required except a ground plate during the process of charging the coated surface. After the charging step the paper can be removed from the ground plate, for exposure and developing. A further advantage is that the preferred zinc oxide utilized is white and the vehicles are colorless so that the surface coating is an excellent medium for the finished print. Coating materials are also relatively cheap and readily available. In the case of Zinc oxides, they are also non-toxic and require no special precautions in handling.

Although the process of the present invention resembles previously proposed electrostatic printing processes to a certain extent, previously proposed processes have not been able to be used to apply a developed image directly to paper or other insulating materials unless some form of electrically conducting backing was used. In the present process, no conductive backing is required after the paper has been given the initial electrostatic charge. Because of this difference it is evident that the coating compositions used as a necessary part of the present process have different properties than those previously proposed for electrostatic printing media.

The coated paper can be prepared in roll form and can therefore be used in a continuous printing process. In this type of process, as the sensitized paper is unrolled, it is conducted along a predetermined path, given a charge, and then exposed to a series of light images either as the paper passes a certain station or a plurality of stations. As the paper continues to travel, the developing powder is applied to the exposed portions such that a series of developed images is produced.

In accordance with the present invention, apparatus suitable for carrying out a continuous printing process is shown in the drawing. Referring to FIG. 5, paper 16, coated with a photoconductive composition as heretofore described, is unrolled from a roll 17 and led past a charging device 18 similar to that shown in FIG. 1. Here, one side of the paper 16a is given an over-all electrostatic charge. The paper, carrying the charge, is then passed across a slit projector 19 with the charged side of the paper preferably, but not necessarily, facing the projector lens.

The slit projector may be of conventional type comprising an enclosed light source 20, condensing lens 21, projection lens 22, and an aperture 23 in the form of a narrow elongated slit. The material to be copied is in the form of a transparent film 24 and the film is passed through the projector by being unwound from a reel 25 and wound on a take-up reel 26. The speed of the take up reel is controlled by conventional variable speed drive means (not shown).

As the paper is exposed to the light images being con-- tinuously projected upon it by the projector, electrostatic latent images are formed as previously described. The electrostatic latent images are then developed by cascading a composition 27 comprising an electroscopic developer powder, which may comprise a fusible resin, and carrier particles fed from a reservoir 28 across the exposed side of the paper as it is led in an inclined path. Particles of the powder adhere either to the charged portions of the image or to the edges of the charged portions, as explained previously. The excess of powder and carrier particles may be collected in a receptacle 29 and recycled through the reservoir 28. The carrier particles may comprise tiny glass beads.

The paper carrying the developed images is then led over a heating device which may comprise a heated drum 30, to fuse and fix the powder making the images permanent. The paper is then passed through drive rollers 31 which may be driven by a variable speed motor (not shown) to control the speed of the paper. The speed of 9 the drive rollers 31 and of the film take-up reel 26 must be synchronized so that the speed of the paper and the speed of the film 24 through the projector are the same.

The entire apparatus may be enclosed to keep out external light or only that part encompassing charging unit, exposure unit and developing unit may be enclosed.

There has thus been described an improved process of electrostatic printing, improved apparatus which may be utilized in the process, and an improved recording medium for use with the process and apparatus.

What is claimed is:

1. In a process of forming a visible image, the steps of providing a recording element comprising a composition comprising a photoconductive Zinc oxide suspended in an electrically insulating, film-forming vehicle for said zinc oxide, forming a pattern of electrostatic charges corresponding to said image on a surface of said composition, and developing said pattern by depositing a finely-divided developer substance on the surface bearing said pattern in configuration conforming with said pattern.

2. In a process of forming a visible image, the steps of providing a recording element comprising a base material carrying a composition comprising Zinc oxide having a surface photoconductivity higher than 10 ohms-V square/watt/cm. at about 3900 A. suspended in an electrically insulating, film-forming vehicle for said zinc oxide, forming a pattern of electrostatic charges corresponding to said image on a surface of said composition, and developing said pattern by depositing a finely-divided developer substance on the surface bearing said pattern in configuration conforming with said pattern.

3. The process of claim 2 in which the ratio of said oxide to said vehicle is between about 1 to l and 12 to 1.

4. The process of claim 3 in which said base material is paper.

5. The process of claim 3 in which said base material is electrically conducting.

6. A process of forming an image comprising providing a cellulosic sheet carrier base having a surface coated with a composition comprising zinc oxide having a surface photoconductivity higher than l ohmr /square/ watt/cm? at about 3900 A. suspended in an electrically insulating, film-forming vehicle for said zinc oxide, applying an over-all electrostatic charge in the dark to at least a portion of said coated surface, exposing at least a part of said charged portion to a light image whereby a pattern of charges constituting a charge image is formed on said coated surface, and developing said charge image by depositing a finely-divided developer substance on the surface bearing said pattern in configuration conforming with said pattern.

7. The process of claim 6 in which said carrier base is paper.

8. The process of claim 6 in which said carrier base is cellophane.

9. The process according to claim 6 in which said photoconductive substance is a white zinc oxide.

10. A process of forming an image comprising providing a recording element comprising a cellulosic sheet base material carrying a composition comprising zinc oxide having a surface photoconductivity higher than ohms- /square/watt/cm. at about 3900 A. suspended in an electrically insulating, film-forming vehicle for said Zinc oxide, forming a pattern of electrostatic charges corresponding to said image on a surface of said composition, and developing said pattern by applying thereto a finely divided electroscopic material.

11. The process of claim 10 in which said electroscopic material is positively charged.

12. The process of claim 10 in which said electroscopic material is negatively charged.

13. A process of forming images comprising providing a carrier sheet of fibrous material having a calendered surface, impregnated with a composition comprising zinc oxide having a surface photoconductivity higher than 10- ohmr /square/watt/cm. at about 3900 A. suspended in an electrically insulating, film-forming vehicle for said zinc oxide, applying a pattern of charges constituting an electrostatic charge image to each side of said sheet, and developing said images by depositing a finely-divided developer substance on the surfaces bearing said patterns in configuration conforming with said patterns.

14. A recording element for electrostatic printing comprising a cellulosic sheet base material carrying a photoconducting insulating layer comprising zinc oxide having a surface photoconductivity higher than 10- ohms-V square/watt/crn. at about 3900 A. suspended in an electrically insulating, film-forming vehicle for said zinc oxide, in which the ratio of said Zinc oxide to said vehicle is between about 1 to l and about 12 to 1, said layer having an applied overall electrostatic charge of such magnitude as to form an electrostatic latent image upon the mere exposure to a light image, which latent image is developable by the application thereto of an electroscopic powder.

15. The recording element of claim 14 in which said vehicle is a synthetic resin.

16. The recording element of claim 14 in which said base material is paper.

17. A recording element for electrostatic printing comprising a base material carrying a photoconducting insulat ing layer comprising white zinc oxide having a surface photoconductivity higher than about 10 ohms /square/ watt/cm. at about 3900 A. suspended in an electrically insulating, film-forming vehicle for said oxide, in which the ratio of said oxide to said vehicle is between about 1 to 1 and about 12 to 1, said layer having an applied overall electrostatic charge of such magnitude as to form an electrostatic latent image upon the mere exposure to a light image, which latent image is developable by the application thereto of an electroscopic powder.

18. The recording element of claim 17 in which said base material is paper.

19. The recording element of claim 18 in which said vehicle comprises a synthetic resin.

20. A process of producing a visible image comprising forming a latent pattern of electrostatic charges corresponding to said visible image upon a photoconducting insulating material comprising zinc oxide having a surface photoconductivity higher than 10 ohms /square/watt/ cm. at about 3900 A. suspended in an electrically-insulating, film-forming vehicle for said zinc oxide, and developing said pattern by applying thereto a finely-divided electrostatically-attractable material.

21. The process of claim 20 wherein said photoconductive substance is a zinc oxide having a surface photoconductivity higher than about 10* ohmr /square/watt/ 0111. at about 3900 A.

22. A recording element for electrostatic printing comprising a base material carrying a photoconducting insulating layer including zinc oxide having a surface photoconductivity higher than about 10- ohms- /square/watt/ cm. at about 3900 A., said zinc oxide suspended in an electrically-insulating, film-forming vehicle, the ratio of said zinc oxide to said vehicle being between 1 to 1 and 12 to 1, said layer having an applied overall electrostatic charge of such magnitude as to form an electrostatic latent image upon the mere exposure to a light image, which latent image is developable by the application thereto of an electroscopic powder.

References Cited in the file of this patent UNITED STATES PATENTS 1,574,358 Beebe Feb. 23, 1926 2,165,840 Lewis et a1 Aug. 15, 1939 2,287,348 Hayden June 23, 1942 2,292,914 Wesch Aug. 11, 1942 2,297,691 Carlson Oct. 6, 1942 2,331,444 Wainer Oct. 12, 1943 (Uther references on following page) 1 UNITED STATES PATENTS 2,494,920 Warrick Jan. 17, 1950 2,551,582 Carlson May 8, 1951 2,554,017 Dalton May 22, 1951 2,555,321 Dalton et a1. June 5, 1951 2,588,569 Picard Mar. 11, 1952 2,599,542 Charlson June 10, 1952 2,624,652 Charlson Jan. 6, 1953 2,624,857 Mager Ian. 6, 1953 2,663,636 Middleton Dec. 22, 1953 2,692,178 Grandadam Oct. 19, 1954 OTHER REFERENCES An Introduction to Luminescence of Solids, Levern;

E2 1950; John Wiley and Sons, Inc., N.Y.; pages 128, 390, 413 and 414 relied upon.

Phosphor-Type Photoconductive Coatings for Continuous Tone Electrostatic Electrophotography, Wainer; 1952; Photographic Engineering, vol. 3, No. 1, pages 1222; originally presented May 24, 1951.

Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, 1923; Longmans, Green and C0., 39 Paternoster Row, London, vol. 4, page 602 relied upon.

Supplement 9 to Raw Materials Index, Natl. Paint Varnish and Lacquer Assoc., Inc., Washington, DC, May 1949, pages 39 and 40.

Mattiello: Protective and Decorative Coatings, vol. II, Wiley (1942), pages 369-376.

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1. IN A PROCESS OF FORMING A VISIBLE IMAGE, THE STEPS OF PROVIDING A RECORDING ELEMENT COMPRISING A COMPOSITION COMPRISING A PHOTOCONDUCTIVE ZINC OXIDE SUSPENDED IN AN ELECTRICALLY INSULATING, FILM-FORMING VEHICLE FOR SAID ZINC OXIDE, FORMING A PATTERN OFELECTROSTATIC CHARGES CORRE- 